1. Field of the Invention
The present invention relates to a driving device, and particularly to a driving device and a driving method, wherein an elelctromechanical transducer such as a piezoelectric element is utilized. The driving device of the present invention is suitably used in the lens-driving mechanism of a camera or the driving mechanism of a precision stage.
2. Description of the Related Art
U.S. Pat. No. 6,016,231 discloses one of the conventional driving devices. The driving device comprises electromechanical transducers such as piezoelectric elements, which change in length (expand or contract) under the application of voltage. The driving device is shown in the exploded perspective view of FIG. 1A and the assembled perspective view of FIG. 1B.
This driving device causes the moving unit 10 to move relative to the base 1, and it is used, for example, as a lens-driving unit for a camera. That is, it becomes possible to move a lens together with the moving unit 10 by coupling the moving unit 10 to the barrel of the lens.
The piezoelectric element 4 which comprises a plurality of piezoelectric plates laminated on one another is fixed at its one end 4a to the base 1 and is fixed at its other end 4b to the 1st end 5a of the rod 5, in the expanding or contracting direction. The rod 5 is slidably supported by the support portions 2 and 3 formed integrally with the base 1.
The moving unit 10 holds the rod 5 between the body 11 thereof and the cap 12 and grasps the rod 5 under an urging force from the spring 13 in approaching direction of the body 11 and the cap 12. In such the manner, the moving unit 10 is frictionally coupled on the surface of the rod 5.
The piezoelectric element 4 is connected to a voltage control circuit (a driving pulse generator), which is not shown. When a predetermined driving voltage having a sawtooth-like waveform is applied to the piezoelectric element 4, the piezoelectric element 4 vibrates, showing substantially the same sawtooth-like displacement (FIG. 2). With this vibration of the piezoelectric element 4, the rod 5 also vibrates showing sawtooth-like amplitudes, in the longitudinal direction thereof. The graph of FIG. 2 is one showing the vibration amplitudes of the rod 5, as well as the vibration amplitudes of the piezoelectric element 4.
This is described in more detail. The piezoelectric element 4 relatively slowly expands at the gentle upward inclination 101 of the 1st waveform 100 during the time interval “A”, where the rod 5 slowly moves in the arrowhead direction “I” in FIG. 1B. Next, the piezoelectric element 4 quickly contracts to the initial length thereof (indicated by the downward inclination 102 of the waveform) during the time interval “B”, where the rod 5 quickly moves in the arrowhead direction “II” in FIG. 1B.
The same motions are repeated, so that the rod 5 vibrates while repeating the slow motion in the direction “I” and the quick motion in the direction “II”. In this way, the rod 5 vibrates drawing the sawtooth-like waveforms comprising gentle and acute inclinations, as shown in FIG. 2.
Note that the force (the frictional coupling force of the moving unit 10 to the rod 5) from the spring 13 of the moving unit 10 is adjusted, such that the moving unit 10 moves together with the rod 5, when the rod 5 slowly moves, and such that the moving unit 10 remains there due to its inertia (or moves less than the rod 5), when the rod 5 quickly moves, as shown in FIG. 3. Accordingly, the moving unit 10 moves relative to the base 1 in the direction “I”, during the rod 5 is vibrating.
When the moving unit 10 is moved in the direction “II” in FIG. 1B, the vibration waveforms of the piezoelectric element 4 and the rod 5 shown in FIG. 2 are inverted, such that a waveform comprises an acute upward inclination and a gentle downward inclination. The moving principle of the moving unit 10 in this case is the same as that described above.
As mentioned above, it is needed to apply the driving voltage having the sawtooth-like waveform to the piezoelectric element. U.S. Pat. No. 5,917,267 discloses some methods for generating such the driving voltage, as described below with reference to FIGS. 4A to 5B.
<<Using a Waveform Generator and an Amplifier (FIGS. 4A to 4C)>>
A sawtooth-like waveform of 8 bits and 0–5 V is generated through the DA conversion by a waveform generator, and is then amplified to 0–10 V by using a power amplifier (FIG. 4A). Thus, a sawtooth-like waveform of 0–10 V for driving is obtained.
FIG. 4B shows a driving voltage waveform for driving the moving unit 10 in the direction “I” in FIG. 3, and FIG. 4C shows a driving voltage waveform for driving the moving unit 10 in the opposite direction “II” in FIG. 3.
<<Using a Constant Current Circuit and a Switch Circuit (FIGS. 5A and 5B)>>
In the digital circuit shown in FIG. 5A, the notations “A” and “D” represent constant current circuits, and the notations “B” and “C” represent switch circuits. The signals shown in FIG. 5B are respectively fed to the terminals “a” to “d” of this digital circuit to thereby alternately operate the constant current circuits “A”, “D” and the switch circuits “B”, “C” to generate sawtooth-like waveforms for use in 0–10 V driving.
As described above, the conventional driving devices have complicated structures and thus cost higher, since the waveform generators and the amplifiers, or the constant current circuits and the switching means, are needed to obtain the driving voltages having sawtooth-like waveforms.
Therefore, an object of the present invention is to provide a driving device including an electromechanical transducer, which is provided with a driving pulse generator which generates a driving voltage having a sawtooth-like waveform with a simple circuit arrangement and which consumes less power.
It is another object of the present invention to provide a driving method for the above-mentioned driving device.